The present invention relates to compensation of Doppler shift in a mobile communication station.
In a mobile communication system, signals which are transmitted from mobile stations moving relative to a base station are subject to the well-known Doppler effect, which causes a frequency shift in the frequency received at the base station relative to that which was transmitted by the mobile station. This frequency shift is referred to herein as the Doppler shift. The Doppler shift is dependent upon the speed and direction of movement of the mobile station relative to the base station. Thus, the Doppler effect can provide an increase or a decrease in the frequency, depending on the direction of movement of the mobile station relative to the base station. The magnitude of the Doppler shift is dependent on the speed with which the mobile station is moving relative to the base station.
Existing mobile communication installations provide a form of Doppler compensation, in that the frequency detection circuitry within the base station which selects a particular signal on a particular channel can take into account a certain amount of Doppler shift in the signal.
A technique for implementing Doppler compensation is described in our earlier Application PCT/EP97/00489.
That technique is implemented by the following steps:
determining a channel impulse response for the channel on which the signal is received;
using the channel impulse response to estimate data bits of a selected portion of the received signal;
generating a reference vector using the channel impulse response and the estimated bits;
determining a Doppler characteristic using the reference vector and the selected portion of the received signal; and
using the Doppler characteristic to provide a Doppler shift compensation for the received signal.
Doppler shift can cause particular difficulties in the detection of signals transmitted from a fast moving mobile. If the Doppler shift can be corrected then the received signal can be detected more accurately.
The present invention relates to an improvement on the above-referenced technique which is intended to provide a Doppler correction method which performs well regardless of the speed of the mobile station. At low mobile velocities there is little Doppler shift therefore correction can degrade performance. However, at high velocities correction is required to improve signal detection. The present inventor has found a compromise so that for high mobile velocities, the receiver performance is improved and conversely at low mobile velocities the performance degradation is minimum.
According to one aspect of the present invention there is provided a method for compensating for Doppler shift in a signal transmitted between a mobile station and a base station in a mobile communication system, the method comprising:
determining a Doppler characteristic for two selected portions of the received signal, the Doppler characteristic being in the form of a phase offset for each selected portion; and
using the larger phase offset as a Doppler characteristic to provide a Doppler shift compensation for the received signal.
According to another aspect of the present invention there is provided a system for compensating for Doppler shift in a signal transmitted between a mobile station and a base station in a mobile communication system, the system comprising:
circuitry for determining a Doppler characteristic for two selected portions of the received signal, the Doppler characteristic being in the form of a phase offset for each portion; and
circuitry for implementing a Doppler shift compensation on the basis of the larger phase offset as a Doppler characteristic for the received signal.
It has been found surprisingly that by always taking the larger phase offset, there is an improvement in signal estimation for the received signal. The selected portions of the received signal may be distinct or overlapping. In the described embodiment, one selected portion is close to the central part of the received signal, and the other selected portion is remote from the central part, but there are many other possibilities for location of the selected portions.
Moreover, it is possible to take more than two selected portions and to take the larger offset over three or more selected portions as the Doppler characteristic used to implement Doppler shift compensation.
The Doppler characteristic may be determined using a reference vector which has been generated using a channel impulse response determined for the channel on which the signal is received and estimated data bits which have been estimated using the channel impulse response. This is in line with the technique described in our earlier Application PCT/EP97/00489.